In this chapter, the author examines what it means to be a member of a state, and discusses how states have conceptualized and institutionalized membership, and how this membership has developed into an international political issue. He covers such topics as the old state orders, religious communities under the state, and the development of the conception of passports. The chapter provides an overview of the development of the nationality systems of the major European countries that are considered the models of modern nation-states, as well as a historical overview of the US nationality system. Since nationality systems determine who belongs to which state, they have international implications, with political friction sometimes generated by the inconsistencies among these systems. Historical case studies include the revolutions in America and France, the differences between French and German systems, and the tightening of British and American immigration regulations.